MADRID, Spain — Suspected Basque rebels detonated a powerful car bomb during rush hour in Madrid on Wednesday, injuring 52 people in the sixth attack since Spain’s prime minister offered the group negotiations if it renounces violence, authorities said.

An anonymous warning on behalf of the separatist group ETA to a pro-independence Basque newspaper preceded the morning blast in an area of office buildings and shops in a working-class neighborhood in Spain’s capital, officials said.

Police had time to cordon off the area before the blast.

The explosion — the first blamed on ETA members in Madrid since February — shattered windows and damaged building facades and cars, but none of the 52 people injured was critically wounded and only five of them were hospitalized, the city’s emergency medical services said.

Opposition conservatives seized on the explosion as evidence the ETA has no interest in negotiating a settlement to a decades-old conflict that has claimed more than 800 lives and said Prime Minister Jose Luis Rodriguez Zapatero’s overture to the group is wrong.

“Anything that keeps them alive, or is an appeal to dialogue or any other kind of way out other than putting them in jail is a big mistake,” said Eduaro Zaplana, spokesman for the Popular Party in Parliament.

Kepa Aulestia, a former ETA member who now opposes the group, said the attack was ETA’s way of building up leverage as it prepares for the possibility of negotiations.

“ETA cannot get down on its knees. At the negotiating table, it has to show up with cards,” he said, predicting more attacks this summer as Spain fills with tourists.

Zapatero, speaking during a scheduled Senate session, condemned the blast and insisted that “the only fate that the terrorist group ETA has is to lay down weapons and dissolve.” He did not mention his offer of talks.

Alonso said ETA remained “alive, active and operative,” despite the arrest of more than 200 suspected members in recent years.

He also said Spanish society was now rife with speculation over whether the government had already begun contacts with ETA — which the government denied — and insisted its fight against the group “would benefit tremendously if we all managed to lower this level of noise.”

The blast was the sixth blamed on ETA since Zapatero announced earlier this month he was willing to hold talks with the separatist group if it renounces violence.

Four bombs exploded at industrial sites in the Basque region on May 15, two days after Parliament endorsed Zapatero’s drive for the first talks with ETA since 1999. Two people were slightly injured. Last weekend another small bomb exploded in the Basque town of Zarauz. No one was hurt.

But ETA has not staged a fatal attack since May 2003, and the government says the group has been weakened by waves of arrests in recent years. Zapatero cites these factors as reasons for trying to launch a peace process even though ETA has not declared a cease-fire.